The overall objective of this project is to fully characterize short and long-term effector and memory humoral and T cell-mediated immune responses (CMI) induced by oral immunization with attenuated S. Typhi live oral vaccines in US subjects, and to identify the cellular and molecular mechanisms that best correlate with protection. Our central hypothesis is that the induction of potent and sustained CMI and humoral immunity at both the local and systemic levels is critical for the development of effective typhoid vaccines, a high public health priority. Specifically, using serum, peripheral blood mononuclear cell (PBMC) and mucosal mononuclear cells (MMC) isolated from terminal ileum biopsies obtained from subjects immunized with one dose of new generation attenuated S. Typhi vaccine candidates or one (low protective efficacy) or four (high protective efficacy) doses of the licensed S. Typhi attenuated vaccine strain Ty21a, we propose to test the following hypotheses: (1) protective HLA class II, classical HLA class la and HLA-E-restricted CMI against S. Typhi depends on immunodominant epitopes derived from S. Typhi antigens. A secondary aim is to evaluate the hypothesis that classical and HLA-E-restricted CD8+ effector responses play complementary roles in protection;(2) the longevity, magnitude and characteristics of CMI to S. Typhi in immunized subjects can be predicted by measuring the magnitude and persistence of specific effector memory (TEM) and central memory (TCM) T cell populations in circulation soon after immunization;(3) the longevity, magnitude and characteristics of antibody responses to LPS and S. Typhi proteins in immunized subjects can be predicted by measuring the avidity of these antibodies in serum, as well as by the magnitude and duration of specific memory B cell populations (BM) in circulation soon after immunization;and (4) that oral immunization with S. Typhi strains elicits the appearance in the gut mucosa of S. Typhi-specific responses mediated by B (e.g., BM) and T (e.g., CTL, Th) cells that correlate with protection. Relevance. The development of improved vaccines to prevent antibiotic-resistant typhoid fever in developing countries is a high global public health priority. The potential use of S. Typhi, the causative agent of typhoid fever, as a bioterror agent has added a new sense of urgency. The goal of this project is to accelerate the development of improved typhoid fever vaccines by uncovering the immunological mechanisms that best correlate with protection in humans.